The present invention relates to a method of forming a porous film used as, e.g., an inter-layer dielectric in a semiconductor integrated circuit device.
As the integration density of a semiconductor integrated circuit has increased, an increased wiring delay time resulting from an increase in wire-to-wire capacitance, which is a parasitic capacitance between metal wires, has presented an obstacle to the implementation of a semiconductor integrated circuit with higher performance. The wiring delay time is a so-called RC delay which is proportional to the product of the resistance of the metal wire and the wire-to-wire capacitance.
To reduce the wiring delay time, therefore, it is necessary to reduce the resistance of the metal wire or the wire-to-wire capacitance.
As a method of reducing the wire-to-wire capacitance, the reduction of the dielectric constant of an inter-layer dielectric formed between the metal wires has been considered. As an inter-layer dielectric having a low dielectric constant, a porous film has been under study as a replacement for a conventional silicon oxide film. It can be said that the porous film is only the film capable of providing a dielectric constant of 2.0 or lower.
In view of the foregoing, there have been proposed various methods of forming porous films.
As a first conventional method of forming a porous film, there has been known one wherein a solution of a siloxane polymer precursor containing a thermally unstable organic component is prepared and coated on a substrate to form a coated film, which is then subjected to a thermal process for decomposing and eliminating the organic component such that numerous fine holes are formed in hollow portions from which the organic component has eliminated.
As a second conventional method of forming a porous film, there has been known one wherein a wet gel is formed on a substrate by coating a silica sol solution on the substrate or by performing CVD and then the condensation reaction of the silica sol is caused in the wet gel, while the volume reduction of the wet gel is suppressed by controlling the speed at which the solvent eliminates from the wet gel, thus forming the porous film.
As a third conventional method of forming a porous film, there has been known a method wherein a solution of silica fine particles is coated on a substrate to form a coated film, which is then sintered such that numerous fine holes are formed between the adjacent silica fine particles.
However, the first conventional method has the problem of higher cost since it is necessary to prepare the solution of the siloxane polymer precursor. Moreover, since the coated film is formed by coating the precursor solution on the substrate, the amount of silanol remaining in the coated film is increased to cause such problems as a degassing phenomenon which is the elimination of moisture or the like in a thermal process step performed subsequently and the degradation of the porous film resulting from the absorption of moisture by the film.
On the other hand, the second conventional method has the problem of higher cost since it requires a special coating apparatus for controlling the speed at which the solvent eliminates from the wet gel. Moreover, since a large number of silanol groups remain on the surfaces of the fine holes, they may cause serious degradation of the film because of high moisture absorption, unless they are removed. It is therefore necessary to silylate the surfaces of silanol groups, resulting in a complicated process. In the case of forming the wet gel by CVD, a special CVD apparatus different from a plasam CVD apparatus used normally in a semiconductor process is also required, which also increases cost.
In accordance with the third conventional method, the diameters of the fine holes formed between the adjacent silica fine particles are determined by a geometric configuration in which the silica fine particles are deposited so that the diameters of the fine particles are increased significantly. Accordingly, it is difficult to adjust the dielectric constant of the porous film to 2 or less.
It is therefore an object of the present invention to solve the forgoing problems at once and allow the formation of a porous film having a dielectric constant of 2 or less in a simple process at low cost.
To attain the above object, a first method of forming a porous film according to the present invention comprises the steps of: depositing an organic-inorganic hybrid film on a substrate by plasma enhanced CVD using a gas mixture of a silicon alkoxide and an organic compound as a reactive gas; and forming a porous film composed of the organic-inorganic hybrid film by performing a plasma process using a plasma derived from a gas containing a reducing gas with respect to the organic-inorganic hybrid film.
In accordance with the first method of forming a porous film, the organic-inorganic hybrid film is deposited by plasma enhanced CVD using the gas mixture of the silicon alkoxide and the organic compound. Accordingly, a low-cost material can be used to deposit the organic-inorganic hybrid film. Since the plasma process is performed by using the reducing gas with respect to the organic-inorganic hybrid film deposited by plasma enhanced CVD, the decomposed organic component eliminates and the numerous fine holes are formed in the hollow portions from which the organic compound has eliminated. This ensures the formation of the porous film composed of the organic-inorganic hybrid film and allows molecular-level control of the diameters of the fine holes in the porous film.
A second method of forming a porous film according to the present invention comprises the steps of: depositing an organic-inorganic hybrid film on a substrate by plasma enhanced CVD using a gas mixture of a silicon alkoxide and an organic compound as a reactive gas; and forming a porous film composed of the organic-inorganic hybrid film by performing a thermal process with respect to the organic-inorganic hybrid film in an atmosphere containing a reducing gas.
In accordance with the second method of forming a porous film, the organic-inorganic hybrid film is deposited by plasma enhanced CVD using the gas mixture of the silicon alkoxide and the organic compound. Accordingly, a low-cost material can be used to deposit the organic-inorganic hybrid film. Since the thermal process is performed with respect to the organic-inorganic hybrid film in the atmosphere containing the reducing gas, the decomposed organic component eliminates and the numerous fine holes are formed in the hollow portions from which the organic component has eliminated. This ensures the formation of the porous film of the organic-inorganic hybrid film and allows molecular-level control of the diameters of the fine holes in the porous film.
In accordance with the first or second method of forming a porous film, there is formed the porous film composed of the organic-inorganic hybrid film deposited by plasma enhanced CVD using the gas mixture of the silicon alkoxide and the organic compound. This obviates the necessity for a precursor solution, which is indispensable to the deposition of an organic-inorganic hybrid film by coating, and allows the deposition of the organic-inorganic hybrid film using a low-cost material. Consequently, the cost of the porous film is reduced.
Moreover, since the organic component in the organic-inorganic hybrid film is eliminated by the plasma process using the plasma derived from the gas containing a reducing gas or by the thermal process performed in the gas atmosphere containing the reducing gas and the fine holes are formed in the hollow portions from which the organic component has eliminated, the fine holes having molecular-size diameters can be formed and the dielectric constant of the porous film is reduced reliably.
Furthermore, since the organic-inorganic hybrid film is deposited by plasma enhanced CVD, the amount of remaining silanol is reduced significantly compared with an organic-inorganic hybrid film deposited by coating, so that moisture generated from the remaining silanol is reduced significantly. This reduces moisture which will eliminate from the porous film in the thermal process subsequently performed as well as various troubles resulting from degassing.
In the first or second method of forming a porous film, the silicon alkoxide is preferably an organic silicon alkoxide represented by the general formula: R1Si(OR2)3 where R1 and R2 are the same or different, each representing an alkyl group or an aryl group. The arrangement ensures the deposition of the organic-inorganic hybrid film by plasma enhanced CVD using the gas mixture of the organic silicon alkoxide and the organic compound.
In the first or second method of forming a porous film, the reducing gas preferably contains a hydrogen gas or an ammonia gas.
In the first or second method, if the reducing gas contains the hydrogen gas or the ammonia gas, a silicon atom remaining after the decomposition and elimination of the organic component is terminated by hydrogen, so that the surfaces of the fine holes formed in the hollow portions from which the organic component has eliminated become hydrophobic. As a result, moisture is prevented from entering the fine holes so that the moisture absorption resistance of the porous film is increased.
If the reducing gas contains the ammonia gas, in particular, the surface of the porous film and the surfaces of the fine holes are nitrided so that the metal composing the metal film deposited to come in contact with the porous film is less likely to be diffused in the porous film, which increases the insulation resistance of the porous film.
A third method of forming a porous film according to the present invention comprises the steps of: depositing, on a substrate, an organic-inorganic hybrid film having a siloxane skeleton; and forming a porous film composed of the organic-inorganic hybrid film by performing a plasma process using a plasma derived from a gas containing a reducing gas with respect to the organic-inorganic hybrid film.
In accordance with the third method of forming a porous film, the plasma process using the plasma derived from the gas containing the reducing gas is performed with respect to the organic-inorganic hybrid film having a siloxane skeleton. Consequently, the decomposed organic component eliminates to leave the numerous fine holes formed in the hollow portions from which the organic component has eliminated. This ensures the formation of the porous film composed of the organic-inorganic hybrid film and allows molecular-level control of the diameters of the fine holes in the obtained porous film, so that the dielectric constant of the porous film is reduced reliably.
In the third method of forming a porous film, the reducing gas preferably contains a nitrogen atom.
In the arrangement, the surface of the porous film and the surfaces of the fine holes are nitrided so that the metal composing the metal film deposited to come in contact with the porous film is less likely to be diffused in the porous film, which increases the insulation resistance of the porous film. In this case, if the reducing gas contains an ammonia gas, the surface of the porous film and the surfaces of the fine holes can be nitrided reliably.
In the third method of forming a porous film, the reducing gas preferably contains a hydrogen atom.
In the arrangement, a silicon atom remaining after the decomposition and elimination of the organic component is terminated by hydrogen so that the surfaces of the fine holes formed in the hollow portions from which the organic component has eliminated become hydrophobic. This prevents moisture from entering the fine holes and increases the moisture absorption resistance of the porous film.
In this case, if the reducing gas contains a hydrogen gas or an ammonia gas, a silicon atom remaining after the decomposition and elimination of the organic component is surely terminated by hydrogen.
A first method of forming a wiring structure according to the present invention comprises the steps of: depositing, on a substrate, an organic-inorganic hybrid film having a siloxane skeleton; forming a resist pattern on the organic-inorganic hybrid film; performing etching with respect to the organic-inorganic hybrid film masked with the resist pattern to form a depressed portion composed of a wire groove or a contact hole in the organic-inorganic hybrid film; performing a plasma process using a plasma derived from a gas containing a reducing gas with respect to the resist pattern and the organic-inorganic hybrid film to remove the resist pattern and form an inter-layer dielectric which is a porous film composed of the organic-inorganic hybrid film; and filling a metal film in the depressed portion of the inter-layer dielectric to form a buried wire or contact composed of the metal film.
In accordance with the first method of forming a wiring structure, the plasma process using the plasma derived from the gas containing the reducing gas is performed with respect to the resist pattern and to the organic-inorganic hybrid film. This allows the step of removing the resist pattern and the step of forming a porous film composed of the organic-inorganic hybrid film to be performed simultaneously. As a result, the porous film composed of the organic-inorganic hybrid film can be formed without increasing the number of process steps.
A second method of forming a wiring structure according to the present invention comprises the steps of: depositing, on a substrate, a first organic-inorganic hybrid film containing an organic component in a relatively low proportion; patterning the first organic-inorganic hybrid film to form a contact hole in the first organic-inorganic hybrid film; depositing, on the first organic-inorganic hybrid film, a second organic-inorganic hybrid film containing an organic component in a relatively high proportion; patterning the second organic-inorganic hybrid film to form a wire groove in the second organic-inorganic hybrid film; filling a metal film in the contact hole and in the wire groove to form a contact and a metal wire each composed of the metal film; and performing a porous-film forming process with respect to the first and second organic-inorganic hybrid films in an atmosphere containing a reducing gas to form a first inter-layer dielectric which is a porous film composed of the first organic-inorganic hybrid film and a second inter-layer dielectric which is a porous film composed of the second organic-inorganic hybrid film.
In accordance with the second method of forming a wiring structure, the amount of the organic component contained in the second organic-inorganic hybrid film is larger than the amount of the organic component contained in the first organic-inorganic hybrid film, so that the porosity of the second organic-inorganic hybrid film is higher than the porosity of the first organic-inorganic hybrid film. As a result, the second inter-layer dielectric is lower in dielectric constant than the first inter-layer dielectric. Moreover, the first inter-layer dielectric is higher in mechanical strength and heat conductivity than the second inter-layer dielectric. Accordingly, a wire-to-wire parasitic capacitance produced between the metal wires formed in the second inter-layer dielectric is reduced and heat generated in the metal wires is diffused efficiently into the substrate via the first inter-layer dielectric. Furthermore, since the wiring structure retains sufficient mechanical strength with the first inter-layer dielectric excellent in mechanical strength, it is possible to reduce the dielectric constant of the inter-layer dielectric, while retaining the sufficient mechanical strength of the wiring structure.
A third method of forming a wiring structure according to the present invention comprises: depositing, on a substrate, an organic-inorganic hybrid film having a siloxane skeleton; patterning the organic-inorganic hybrid film to form a wire groove in the organic-inorganic hybrid film; filling a metal film in the wire groove to form a buried wire composed of the metal film; and performing a plasma process using a plasma derived from a gas containing a reducing gas with respect to the organic-inorganic hybrid film to form an inter-layer dielectric which is a porous film composed of the organic-inorganic hybrid film.
In accordance with the third method of forming a wiring structure, the inter-layer dielectric composed of the porous film is formed by forming the buried wire and then performing the plasma process using the reducing gas with respect to the organic-inorganic hybrid film, so that the metal composing the buried wire is prevented from entering the fine holes in the inter-layer dielectric. This prevents an increased leakage current and a short circuit between the buried wires. Since it is unnecessary to form a protecting film in the wiring groove, the wire-to-wire parasitic capacitance between the buried wires can be reduced reliably.
A wiring structure according to the present invention comprises: a first inter-layer dielectric formed on a substrate and having a contact hole, the first inter-layer dielectric being composed of a porous film having a relatively low porosity; a second inter-layer dielectric formed on the first inter-layer dielectric and having a wire groove, the second inter-layer dielectric being composed of a porous film having a relatively high porosity; a contact composed of a metal film filled in the contact hole; and a metal wire composed of a metal film filled in the wire groove.
In the wiring structure according to the present invention, the porosity of the second inter-layer dielectric is higher than the porosity of the first inter-layer dielectric, so that the second inter-layer dielectric is lower in dielectric constant than the second inter-layer dielectric and higher in mechanical strength and heat conductivity than the first inter-layer dielectric. As a result, it is possible to reduce the dielectric constant of the inter-layer dielectric and retain the sufficient mechanical strength and heat diffusing property of the wiring structure.